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Tropical Storm IDA Forecast Discussion Number 16
2015-09-22 10:37:55| National Hurricane Center (Atlantic)
Issued at 500 AM AST TUE SEP 22 2015 000 WTNT45 KNHC 220837 TCDAT5 TROPICAL STORM IDA DISCUSSION NUMBER 16 NWS NATIONAL HURRICANE CENTER MIAMI FL AL102015 500 AM AST TUE SEP 22 2015 Shortwave infrared satellite imagery indicates that Ida's low-level circulation center is now moving or developing east-southeastward closer to the large mass of deep convection that has been persisting in the southeastern portion of the larger circulation. The initial intensity is being maintained at 40 kt, especially now that the low-level center has moved closer to the mid- and upper-level circulations as seen in microwave and conventional satellite images. This intensity is supported by a Dvorak current intensity estimate of T3.0/45 kt from TAFB. The initial motion estimate is now 105/04 kt. The recent and much anticipated turn to the east-southeast appears to have occurred either due to actual storm motion or due to redevelopment of the low-level center closer to the strong convection. Regardless of the cause in the Ida's recent motion change, the global and regional models are in very good agreement on a continued general slow motion toward the east for the next 48 hours or so as the southern portion of a large mid- to upper-level trough moves across and captures the cyclone. By 72 hours, the trough lifts out to the northeast and releases Ida, allowing the cyclone to move slowly toward the northwest by day 4 and toward the north on day 5. As would be expected in such a weak flow regime, the model guidance is widely divergent after 72 hours with the UKMET, HWRF, and NAVGEM models taking Ida more toward the west-northwest, whereas the GFS, ECMWF, GFDL, and Canadian models take Ida more toward the northwest and north. The one thing that the all of the models do agree on, however, is that Ida is not expected move very quickly during the next 5 days. The NHC track forecast is similar to but slower than the previous advisory track, and lies close to a blend of the consensus models TCVA and GFEX. Given the uncertainty in exactly when and where the low-level center of Ida will move underneath the mid/upper-level circulation due to fluctuations in the deep-layer vertical shear profiles during the next 3 days, the intensity forecast calls for no significant changes in the strength of the cyclone during that time. By days 4 and 5, however, some gradual strengthening is expected as the vertical shear abates somewhat while Ida is moving over 29C sea-surface temperatures. The official intensity forecast is similar to the previous advisory, and closely follows the IVCN intensity consensus model. FORECAST POSITIONS AND MAX WINDS INIT 22/0900Z 21.3N 48.6W 40 KT 45 MPH 12H 22/1800Z 21.3N 48.1W 40 KT 45 MPH 24H 23/0600Z 20.9N 47.3W 40 KT 45 MPH 36H 23/1800Z 20.6N 46.7W 40 KT 45 MPH 48H 24/0600Z 20.8N 46.4W 40 KT 45 MPH 72H 25/0600Z 22.0N 46.7W 40 KT 45 MPH 96H 26/0600Z 23.0N 48.0W 45 KT 50 MPH 120H 27/0600Z 25.2N 48.5W 50 KT 60 MPH $$ Forecaster Stewart
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